Vehicle entry and ignition systems that use a unique, physical key to enter and start the vehicle engine are well known in the art. Also, electrical fobs that wirelessly transmit an electrical signal such as a radio frequency (“RF”) signal for controlling locking and unlocking functions of the vehicle doors are known. Recently, passive keyless systems have been developed for vehicles that include a fob carried by a vehicle operator that control door lock/unlock functions and engine immobilization automatically in response to the presence of the fob. As the operator approaches the vehicle and touches a portion of the vehicle, such as the door handle, the vehicle lock/unlocking system automatically sends an interrogation signal to the fob and monitors for a fob response signal. Once identification and authentication occurs between the vehicle system and fob, and if authentication is accomplished, the door is unlocked. Once inside the vehicle, if the fob carried by the operator is the correct fob associated with that vehicle, the vehicle can be started with a simple pushing of a start button located on the vehicle dash, i.e., no form of physical key is needed to start the vehicle. Such systems have become known as “keyless-go” ignition systems.
As part of the authentication process between the vehicle system and the fob, identification (“ID”) codes are sent and compared. For example, the fob may send a coded ID signal via RF to an on-board vehicle system. The on-board vehicle system determines if the received ID code is a valid ID from a fob associated with that vehicle. If the ID code is a valid code, as determined via a comparison against a prestored code, the vehicle will be able to start when the start button is pushed. If the received ID code is not valid, i.e., the received code does not match the prestored code, the vehicle engine is immobilized.